test-lib: tests skipped by GIT_SKIP_TESTS say so

We used to show "(missing )" next to tests skipped because they are
specified in GIT_SKIP_TESTS.  Use "(GIT_SKIP_TESTS)" instead.

Plus tests that check basic GIT_SKIP_TESTS functions.

Signed-off-by: Ilya Bobyr <ilya.bobyr@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/t/test-lib.sh b/t/test-lib.sh
index c081668..e7d9c51 100644
--- a/t/test-lib.sh
+++ b/t/test-lib.sh
@@ -452,25 +452,28 @@
 
 test_skip () {
 	to_skip=
+	skipped_reason=
 	if match_pattern_list $this_test.$test_count $GIT_SKIP_TESTS
 	then
 		to_skip=t
+		skipped_reason="GIT_SKIP_TESTS"
 	fi
 	if test -z "$to_skip" && test -n "$test_prereq" &&
 	   ! test_have_prereq "$test_prereq"
 	then
 		to_skip=t
-	fi
-	case "$to_skip" in
-	t)
+
 		of_prereq=
 		if test "$missing_prereq" != "$test_prereq"
 		then
 			of_prereq=" of $test_prereq"
 		fi
-
+		skipped_reason="missing $missing_prereq${of_prereq}"
+	fi
+	case "$to_skip" in
+	t)
 		say_color skip >&3 "skipping test: $@"
-		say_color skip "ok $test_count # skip $1 (missing $missing_prereq${of_prereq})"
+		say_color skip "ok $test_count # skip $1 ($skipped_reason)"
 		: true
 		;;
 	*)